1. Field of the Invention
The present invention relates to a chemical mechanical polishing (CMP) apparatus used for manufacturing a semiconductor device and a polishing cloth for use with the apparatus. The invention relates particularly to a CMP apparatus for minimizing deterioration in the polishing performance of the polishing cloth and allows easy detection of its useful operational limit.
2. Description of the Related Art
Today as the number of layers of large scale integrated circuits (LSI) and the density of such circuits increases, the use and development of improved polishing and smoothing techniques for interlayer insulation films becomes critically important. At present, chemical mechanical polishing (CMP) is widely recognized for smoothing and final preparation of a semiconductor wafer.
FIG. 5 shows the general configuration of a CMP apparatus. This apparatus comprises at least a rotatable ring 3 for fixing and rotating a semiconductor substrate 1, a polishing cloth 5 for polishing the surface of the semiconductor substrate 1 and a turntable 7 for fixing and rotating the polishing cloth 5. During the formation of a semiconductor substrate, surface irregularities are usually created which need to be removed. After semiconductor substrate 1 is received, rotatable ring 3 forces surface 1 against cloth 5 and ring 3; turntable 7 is then rotated and abrasive fluid 9 is supplied. As a result, the surface of the semiconductor substrate 1 is smoothed by mechanical polishing and chemical reactions.
For effective polishing, the cloth must have an abrasive quality. The CMP apparatus utilizes a process called dressing to maintain the polishing performance of the polishing cloth. Dressing restores the abrasiveness of a dull polishing cloth. The cloth becomes dull as it is used; its dullness is proportional to the number of times it is used on semiconductor substrates. FIG. 6 shows the operation of a dresser during the dressing process. Dresser 11, containing a diamond granular surface, is pressed against the surface of the polishing cloth 5 fixed to turntable 7; dresser 11 and polishing cloth 5 respectively rotate during the dressing operation. Alternatively, the dresser 11 itself may be moved horizontally in order to completely cover the surface of the polishing cloth.
FIG. 7 is an enlarged view of dresser 11. An abrasive diamond granular surface 13 is formed by a mixture of abrasive diamond grains on an annular region of dresser 11. Diamond grain surface 13 is pressed against the surface of the polishing cloth during the dressing. In accordance with the invention described below, dressing may be performed simultaneously during the semiconductor substrate polishing. Alternatively, it may be performed before or after the substrate polishing.
FIG. 8 is a diagram showing the practical configuration of a general CMP apparatus. The surface of polishing cloth 5 has an abrasive surface comprising, for example, dimples or lattice grooves for distributing an abrasive fluid 9 along the entire surface of polishing cloth 5. As the polishing cloth processes more and more area of substrate 1, the cloth becomes worn and thin. When the wear on the polishing cloth 5 exceeds a particular limit, both the polishing speed and the uniformity of the polishing will deteriorate. Therefore, a test board, made of a sample semiconductor substrate, has been used in the prior art to detect the wear of the cloth; it assists in determining whether it has reached a use limit indicating the cloth is no longer effective for polishing. The polishing speed and the surface polishing uniformity are calculated during this inspection process; accordingly, it can be determined whether the polishing cloth 5 has exceeded its use limit. Upon determining that the cloth has not reached the use limit, the CMP apparatus can then be used to polish substrate 1; polishing cloth 5 is replaced, however, upon exceeding its use limit.
During dressing, abrasive diamond grains may fall off the diamond surface and onto the cloth; as a result, the fallen grains may damage the surface of the semiconductor substrate during subsequent polishing operations. FIGS. 9 is a cross section of the abrasive diamond granular surface 13 shown in FIG. 7. As shown in FIG. 9(a), diamond grains 17 are embedded in a nickel layer 19 of surface 13 for dressing the polishing cloth. As shown in FIG. 9(b), friction created by the contact of surface 13 and the polishing cloth 5 during dressing causes diamond grains 17 to fall off nickel layer 19 and drop onto cloth 5. As shown, nickel layer 19 becomes thinner as it is scoured during dressing and grains 17 fall from the surface. Some diamonds are more susceptible to loosening and falling because their area contacting nickel layer 19 is small and thereby may be more easily removed during the dressing operation. As a result, diamond grains 17 fall off continuously. The presence of extraneous fallen diamond grains on the polishing cloth will potentially destroy the substrate during the polishing step. In addition, the use of a test board has associated problems. First, the use of test boards necessarily results in a waste of semiconductor substrates since they must be abandoned after their temporary use. Second, since the size of the test board substrate must be commensurate with the semiconductor substrate used in production, the subsequent discarding of the board results in further waste and costs. Further, the time required for test board processing and evaluation is problematic counterproductive in attempting to improve manufacturing efficiency in a production line.